Orifice fitting devices are useful in measuring the flow of fluids in pipelines. An orifice fitting commonly includes, among other things, an orifice plate, which has a restricted bore. As is well known, where a fluid flows through a restricted orifice in a pipeline, a pressure differential is developed across the upstream and downstream sides of the orifice. This pressure differential can be detected and measured. Indeed, the pressure differential, together with other factors such as pressure, type of fluid, temperature and so forth, can be used to calculate the amount of the fluid which flows through pipeline during the given time period.
As is well known, orifice fittings can be used to measure, among other things, the flow of natural gas through gas pipelines. In fact, orifice fittings are used as the primary method of measuring natural gas as it flows through pipelines throughout the world. Orifice fittings of a type heretofore used in natural gas pipelines include a square orifice plate carrier that has a recess in it that accepts an orifice plate and a seal. In a dual-chambered orifice fitting, the plate carrier assembly (i.e., the combination of the orifice plate carrier, the orifice plate, and a seal), can be lowered into or removed from its position in the flow passage of the orifice fitting by an operator. The plate carrier assembly's position within the fitting (and more specifically, within the flow passage of the fitting) of this type of orifice fitting is held by three fixed-in-place screws. An operator typically centers the plate carrier assembly in the vertical direction by adjusting one of the three screws and then adjusting the other two screws for horizontal centering. This is done by a trial and error method where the operator puts the carrier assembly in the fitting, adjusts the screws, takes a measurement, and then readjusts the screws. This process is typically repeated until the orifice plate's bore is centered within the flow passage of the orifice fitting.
A variety of orifice fittings, orifice plates, and orifice plate carriers have been suggested over the years. Typical prior art orifice fittings, orifice plates and orifice plate carriers, as well as other devices, are those shown in U.S. Pat. Nos. 3,817,287, 4,444,224, 4,478,251, 4,593,915, 4,633,911, 4,750,524, 5,042,531, 5,094,272, and the 1990 brochure entitled "Superior Dual Chamber Orifice Fitting," published by Superior Measurement Equipment of Houston, Tex., all of which are incorporated herein by reference.
U.S. Pat. No. 3,817,287 describes an orifice fitting with a saddle for receiving an orifice plate and seal and allows installation and removal of orifice plates without the necessity of depressurizing the line to which the orifice fitting is attached. U.S. Pat. No. 4,444,224 describes another orifice plate mechanism that allows installation and removal of orifice plates without the necessity of depressurizing the line to which the orifice fitting is attached. This patent also describes the use of an ejector plate, which supports the orifice plate between gear operated seat members. The gear operated seat members hold the orifice plate in place in the flow passage during operation of the orifice fitting; the orifice plate is inserted, held in place and removed via a relatively complicated system of gears and cams and requires the use of a specially designed orifice plate. U.S. Pat. No. 4,478,251 describes orifice plates and specifically, an orifice fitting seal assembly comprising internal seal rings, an external locking ring, an outer pair of O-ring sealing elements, and an inner pair of O-ring sealing elements. U.S. Pat. No. 4,593,915 describes an orifice plate seal ring for use in an orifice fitting, comprising two seal rings composed of sufficiently hard material as to resist swelling or distortion under high pressures and/or high temperatures. U.S. Pat. No. 4,633,911 describes an orifice plate seal ring having a U-cup spring located within circumferential grooves in the seal ring, where the seal ring completely encases the outer circumference of the orifice plate. U.S. Pat. No. 4,750,524 describes an apparatus for changing orifice plates in orifice fittings, but requires a specially designed rectangular parallelepipedal orifice plate. U.S. Pat. No. 5,042,531 describes an energized seal for use in orifice fitting wherein a hollow pin containing a piston mounted on the orifice plate carrier is used to energize the seal. U.S. Pat. No. 5,094,272 describes an orifice plate seal which can be adjusted to allow the desired amount of interference fit between the orifice plate carrier and the seat slot.
Large commercial transactions, such as sales of natural gas, are based on the data generated by measurements made with the use of orifice fittings. Because of this, orifice fittings generally and, more particularly, orifice plates, have been studied in detail. To bring uniformity and accuracy to such measurements, various organizations have promulgated written standards dealing with orifice plates.
Industry standards require "concentricity"; i.e., they require that the center of the orifice plate's bore and the center of the flow passage (as defined by the upstream and downstream inside walls of the flow passage) to be within certain tolerance limits. This is because the orifice or bore defined by the orifice plate must be "centered" within the flow passage of the fitting to ensure accurate measurement of the pressure differential. To be centered, the orifice plate's bore must be concentric with the inside wall of the flow passage of the fitting. If the orifice plate's bore were perfectly concentric within the flow passage of the fitting, the center of the orifice plate bore would be the same distance, in all directions, from both the upstream and downstream inside walls of the fitting. Deviations from perfect concentricity of the orifice plate bore are normally termed "eccentricity" by natural gas industry standards for orifice plates. As used herein, "eccentricity" is used to describe the amount of a deviation from perfect concentricity.
Until fairly recently, most standards allowed the orifice plate to be centered within the pipeline with somewhat liberal tolerance. In 1980, a new International Standard, ISO-5167, was promulgated. This standard drastically reduced the preexisting tolerances. Because of these new tolerances, this new standard focused new attention on the centering of the orifice plate. More recently, revised standards with revised tolerances for centering an orifice plate's bore have been issued. An example of such a revised standard is the standard set forth in Chapter 14, Section 3 of the 1991 version of the API Manual of Petroleum Measurement Standards, which is incorporated by reference herein. In conjunction with the revised standards for tolerances of "eccentricity," more rigorous industry standards have been imposed on the maximum allowable eccentricity of the bores of orifice plates. For example, Chapter 14, Section 3 of the 1991 version of the API Manual of Petroleum Measurement Standards sets a revised standard for concentricity that has been accepted by the American Gas Association, the Gas Processors Association and as an American National Standard (ANSI). Similar centering requirements set forth in the above standards have also been adopted by a revised International Standard ISO-5167, formally entitled "Measurements of Fluid Flow by Means of Pressure Differential Devices, 1991 Edition," thus establishing world-wide acceptance of similar standards for centering an orifice plate bore within an orifice fitting's flow passage.
Such current industry standards provide a maximum eccentricity value for deviations from perfect concentricity in both the horizontal and vertical directions. The "horizontal" direction, as used herein, is defined for convenience as the direction parallel to the longitudinal axis of the flow passage and parallel to the plane of the pressure taps used to measure the pressure differential, as shown by the arrow labeled "H" in FIG. 2. The "vertical" direction, as used herein, is defined for convenience as the direction perpendicular to the longitudinal axis of the flow passage and perpendicular to the plane of the pressure taps, as shown by the arrow labeled "V" in FIG. 2. Hence, references herein to "up," "down," "top," "bottom," and the like will be understood to follow the convention for "vertical" and "horizontal" noted above. These definitions follow the usage of the API Standards discussed above. Of course, those skilled in the art will understand that an orifice fitting may need to be installed in an unusual position because of surrounding conditions, such as a pipe or the like, or to facilitate the measurement of moist gases. In such cases, terms such as "vertical" and "horizontal" and the like will still be understood in reference to the directions as defined above.
One method of determining eccentricity values is described in Chapter 14, Section 3, Paragraph 2.6.2.1 of the 1991 version of the API Manual of Petroleum Measurement Standards. In this method, the perpendicular distance between the edge of an installed orifice plate's bore and the inside wall of the pipeline is measured at the location of a pressure tap, a well known device typically used to measure the pressure differential in an orifice fitting. Typically, the pressure differential is measured by the use of two pressure taps, one located on the upstream side of the orifice plate and one located on the downstream side of the orifice plate. As is well known in the art, these pressure taps are aligned in the same plane as the direction of flow of the fluid, parallel to the longitudinal axis of the flow passage and each extends inward through the flow passage of the orifice fitting to the flow of fluid.
The same measurements are then taken on the opposite side of the flow passage, 180 degrees from the location of the first measurement. One half of the difference between the two measurements represents the eccentricity value in the horizontal direction. Two similar measurements ate then taken in the vertical direction. (This method assumes that the orifice plate's bore and the inside wall of the pipeline are round to within a tolerance smaller than the maximum allowable eccentricity value.) One means of making such eccentricity measurements is by the use of an eccentricity gauge, such as the TCS Eccentricity Gauge marketed by TCS Sales of Bossier City, La. Other technically valid techniques for measuring eccentricity are also acceptable under this API standard and are well known to those skilled in the art.
Very small deviations from concentricity in the horizontal direction can cause relatively large errors in the pressure differential measurement. The pressure differential measurement is not as sensitive to deviation from concentricity in the vertical direction. Therefore, industry standards generally impose tolerance limits in the horizontal eccentricity value that are more stringent than the tolerances allowed in the vertical direction. For example, the standard set forth in Chapter 14, Section 3 of the 1991 version of the API Manual of Petroleum Measurement Standards imposes a horizontal eccentricity tolerance four times tighter than its vertical tolerance. The problems in maintaining acceptable values of eccentricity in the horizontal direction are exacerbated by the fact that orifice fitting devices are sometimes installed in a sideways position to facilitate flow measurement in moist gases. When the orifice fitting is installed horizontally, the horizontal direction (as previously defined herein) is perpendicular to the ground, and thus, gravity can contribute to or exacerbate existing deviations from concentricity in the horizontal direction.
One way in which eccentricity tolerances have been met (i.e., the orifice plate bore has been centered) in the past has been by manufacturing the orifice plate, the orifice plate's bore, the orifice plate carrier, and the seals to extremely tight tolerances, and then manually positioning the orifice plate carrier in place. The orifice plate carrier was typically positioned through the use of adjustable set screws. The set screws would be manually adjusted by an operator in response to repeated measurements to determine eccentricity, until the centering of the orifice plate carrier was accomplished. The set screws would then be welded in place.
One disadvantage of this technique is that the tolerances associated with the manufacture of the orifice plate, the orifice plate's bore, the orifice plate carrier, the seals, the clearance required between the plate carrier and the set screws, and the inside diameter of the orifice fitting are cumulative. As is well known in the art, where there is a "stacking" of tolerances, the resulting assembly may not meet the applicable requirements even though each of the individual components meet its own rather restrictive tolerance. Hence, even though each of the components in a typical plate carrier assembly might fall within the specified tolerances, the horizontal eccentricity value of such assemblies would still exceed the industry standard.
Further, the industry standards do not set a standard for the outer diameter of the orifice plate, or the tolerance limits within which this dimension must be maintained. This fact introduces more uncertainty into the centering of the orifice plate carrier assembly, since centering the entire orifice plate carrier assembly to center the orifice plate's bore is dependent upon the tolerances of the constituent parts of the orifice plate carrier assembly.
Additional difficulties arise from other components. For example, the seal is typically made from an elastomeric material. Such materials are often difficult to mold to the restrictive tolerances required. The seal tends to swell over its lifetime. Since such swelling is not always perfectly uniform, it introduces more error into the initial centering process. Also, because the seals used in typical orifice plate carrier assemblies completely surround the outer circumference of the orifice plate, they prohibit movement of the orifice plate. As a result, the orifice plate and therefore, the orifice plate's bore, often must be centered by aligning the orifice plate within the seal, then the seal within the plate carrier, and then the plate carrier within the fitting, rather than by centering the orifice plate itself. Hence, the bore of the orifice plate was centered within the pipeline bore only indirectly, not directly. Moreover, this process involved a substantial amount of effort by human operators.